1. Field of the Invention
The present invention pertains to the field of medical syringes and more particularly concerns elastomeric drivers for depressing the plunger of resistance syringes used for accurately positioning an epidural needle in preparation for injection of anesthetic into the epidural space of the spinal cord.
2. State of the prior Art
Epidural anesthesia involves the injection of a liquid anesthetic substance into the epidural space which surrounds the dura mater which in turn surrounds the spinal cord proper. The epidural space is defined between the ligamentum flavum on the posterior or back side of the spinal cord, and the anterior longitudinal ligament on the anterior or frontal side of the spinal cord. These flexible but tough ligaments interconnect the bony vertebrae which enclose and protect the spinal cord and spinal canal.
A dose of an anesthetic such as lidocaine or bupivacaine, by way of example, produces a regional nerve block suitable for surgical procedures to be performed on portions of the anatomy affected by the nerve block, primarily but not limited to portions of the anatomy below the level of the spinal column at which the anesthetic is injected.
In the preferred midline technique the epidural needle passes through the supraspinous, interspinous and ligamentum flavum structures before entering the epidural space. Insertion of the needle into the epidural space is complicated by the lack of feedback as to the position of the needle tip, coupled with the imperative need to avoid puncturing the dura mater which surrounds the spinal cord, since there is potential for catastrophic trauma to the spinal cord with the epidural needle. Extreme caution must therefore be exercised in the positioning of the needle tip, which must pierce through the tough, resilient, leather-like ligamentum flavum and then stop immediately within the narrow epidural space, short of puncturing the dura mater.
The needle must be moved through the ligamentum flavum very slowly and in a carefully controlled fashion. At the same time, pressure is applied to the plunger of the attached syringe which is filled either with air or saline solution. The object is to continuously test for loss of resistance to injection, experienced when the needle lumen enters the epidural space after clearing the ligamentum flavum. This loss of resistance is experienced by little if any resistance to injected air or fluid, and a negative aspiration test then indicates that the needle lumen is properly positioned in the epidural space. Special syringes, known as loss of resistance syringes and characterized by very low friction between the plunger and the barrel of the syringe, are used for positioning the needle lumen in the epidural space. Once correct positioning of the needle is achieved, the resistance syringe is separated from the epidural needle and another syringe, loaded with the anesthetic is attached, after which the anesthetic is injected.
In order to appreciate the contribution being made by the present invention, it is important to understand the demands placed upon the anesthesiologist's dexterity by this procedure. It is of critical importance that the needle traverse the ligamentum flavum in a carefully measured and controlled manner. Typically, this is achieved by applying resistance to the advancing needle with the anesthesiologist's non-dominant hand (the left-hand if the anesthesiologist is right-handed) while the dominant hand applies pressure to the plunger of the resistance syringe to test for resistance to injection while at the same time slowly advances the needle. Variations of this technique may be adopted according to personal preference, for example the needle may be advanced continuously while pressure on the syringe barrel is also maintained continuously to test for resistance. In the alternative, the needle is advanced in very small increments, e.g. 1 millimeter, testing for resistance to injection after each advance.
The difficulty of correctly positioning the needle lumen in the epidural space has spurred many attempts to develop methods and devices for detecting and indicating correct needle placement. These expedients have generally exploited the low resistance to injection and subatmospheric pressure characteristic of the epidural space. One such technique involves placement of a drop of saline solution on the open hub of a epidural needle. The drop will be "sucked-in" as the needle lumen enters the epidural space where, for reasons not well understood, prevails sub-atmospheric pressure. Other means used for this purpose include capillary attachments with fluid indicators developed by Odom, or inflated balloons by Macintosh, which deflate upon entering the epidural space. It is also known to use spring loading devices to facilitate the loss of resistance phenomena which occur as the epidural needle passes from the dense ligamentum flavum into the lesser resistance of the epidural space.
U.S. Pat. No. 5,024,662, in which this applicant is a co-inventor, describes an attachment for a resistance syringe for aiding the anesthesiologist in correct placement of the epidural needle. The patented attachment has an elastomeric band retained to the syringe barrel by a ring which slides onto the syringe barrel against the finger flange of the syringe to anchor the ends of the elastic band to the barrel while a midportion of the band is pulled by the plunger of the syringe. Consequently, the plunger is urged by elastic force into the syringe barrel, but is held back by fluid, air or liquid in the barrel, until the needle lumen enters the epidural space. At that point the contents of the syringe are injected into the epidural space under the force of the stretched band, providing the anesthesiologist with immediate kinesthetic indication of correct needle placement.
While this arrangement works well, disposable elastomeric drivers have been developed by this applicant which are of still greater simplicity and very low cost.